Cement flow control tool

ABSTRACT

An apparatus for controlling the flow of fluid into a borehole through a conduit has a decelerating means adapted to be positioned within the conduit. The fluid could typically comprise drilling mud and/or cement, and in some embodiments, some of the cement can exit through apertures in a shroud of the apparatus to cement the decelerating means inside the conduit.

The present invention relates to a cement flow control tool andespecially but not exclusively, a cement flow control tool for use incementing a string of tubulars such as a casing or liner string into anoil, gas or water borehole.

Primary cementing is the process of placing cement in the annulusbetween a casing or liner string and the formations exposed to theborehole. A major objective of primary cementing is to provide zonalisolation in the borehole of oil, gas, and water wells, i.e. to excludefluids such as water or gas in one zone from oil in another zone. Toachieve this, a hydraulic seal must be obtained between the casing andthe cement, and between the cement and the formations, while at the sametime preventing fluid channels in the cement sheath. Without completezonal isolation, the well may never reach its full producing potentialand remedial work to repair a faulty cementing job may do irreparableharm to the producing formation. In consequence, reserves may be lostand commencement of production may be delayed.

After drilling the well to the desired depth, the drillpipe is removedand a casing string is run in until it reaches the bottom of theborehole. The casing string typically has a shoe, such as a float shoe,guide shoe or a reamer shoe on the end to guide the casing string intothe borehole. At this time, the drilling mud (used to remove formationcuttings during the drilling of the well) is still in the borehole; thismud must be removed and replaced by hardened cement.

This is done by passing cement down through the inside of the casingstring; the cement passes out of apertures in the shoe and into theannulus between the borehole and the casing. The drilling mud isdisplaced upwards and the cement replaces it in the annulus. The cementneeds to extend at least as far up the annulus so as to span theproduction zones, and the previous casing shoe if present, and sometimesthe cement even extends to the surface.

However, the cement is heavy and so exerts a large force on the drillingmud. Drilling mud is less heavy than cement, so the cement causes thedrilling mud to travel quickly up the annulus. Fast flowing drilling mudbrings a high pressure to bear upon the formation and excess solids anddrill cuttings may build up in the annulus, exerting even more pressureon the formation. The formation may break down under the pressure,resulting in both severe mud loss and also a loss of production. Openhole sections of the formation are especially prone to collapse,possibly ruining the borehole.

An additional problem is that the cement, being heavier, may also falldown through the drilling mud, resulting in a poor cement job.

According to the present invention there is provided apparatus forcontrolling the flow of cement into a borehole through a conduit, theapparatus comprising a decelerating means adapted to be positionedwithin the conduit for slowing down the flow of fluid through theconduit.

The deceleration means typically controls or mitigates the free falleffect of the cement.

Preferably, the conduit is a drillpipe, tubing, coiled tubing,filtration screen, casing or liner string, but may be any conduit whichis inserted into a borehole.

Typically, the decelerating means comprises a passage, and mostpreferably, the passage is defined by at least one body member havingformations thereon.

Typically, the passage is inclined relative to the axis of the conduitand deceleration of the fluid is caused by friction between the fluidand the inclined passage. Typically, the passage is also inclinedrelative to a plane perpendicular to the axis of the conduit.Optionally, the inclination of the passage is continual throughout itslength.

Typically, the inclined passage has constant dimensions and theboundaries of the passage are free of obstructions so that the fluidmoves along the passage without hindrance.

The passage typically comprises portions with axial and transaxialcomponents, so that the length of the passage is greater than the lengthof the apparatus.

The transaxial components of the passage typically cause the path offluid flowing through the apparatus to deviate from its former axialpath through the conduit prior to flowing through the apparatus, therebydecelerating the fluid.

Preferably, the decelerating means further comprises at least one spiralpassage defined by the at least one body member.

The angle of the spiral portion of the passage is typically more than 60degrees relative to the conduit axis, preferably between 70 and 80degrees and most preferably around 75 degrees.

Preferably, the passage is uni-directional in the axial direction, sothat in use, when fluid is flowing from the top to the bottom of theinternal passage, no part of the passage would direct fluid up theapparatus.

Uni-directional embodiments have the advantage over other designs whichinclude passages having upwardly-inclined portions and correspondingtroughs, in which any suspension would be inclined to settle and blockthe passage.

Such uni-directional embodiments include those having a spiral passage;the continual slope of the spiral passage ensures that gravity canassist the flow of fluid through the passage. Embodiments incorporatingthe spiral design have the advantage that any suspended particlescarried by the fluid will not settle in the passage and block thepassage.

Optionally, the passage includes at least two portions spiralling inopposite directions to each other. Optionally, the spiral passageincludes at least two of said portions and preferably oppositelydirected spiralling portions are positioned adjacent one another.

Preferably, the passage includes two or more of said portions and mostpreferably, the passage is formed so that fluid travelling through afirst portion will flow in a clockwise direction through the spirallingparts of that portion, and fluid travelling through a second,neighbouring portion will flow in an anti-clockwise direction throughits spiralling portion, or vice versa.

Typically, the decelerating means induces turbulence into the fluid todecelerate the fluid.

Optionally, the turbulence is wholly, mainly or partly induced by adirection-altering means, which changes the direction of fluid flowingin the internal passage. Typically, the direction-altering meanscomprises a cavity provided between first and second oppositely directedspiral passage portions, providing a space in which the fluid changesdirection between the first spiral direction and the second spiraldirection. The cavity is typically formed in the at least one bodymember and may comprise a connecting passage linking the spiral passageportions; the connecting passage may include axial portions andtransaxial portions.

Whether turbulent or laminar flow results depends (among otherparameters) on the speed of the fluid through the passage. Thus, inembodiments of the invention which induce turbulence, the apparatus canhave a decelerating effect on some fluids but not on others, dependingon the speed of the fluid. The turbulence will only have a significanteffect upon fast flowing fluids and slow flowing fluids will not beappreciably slowed.

However, simple embodiments of the invention, which may comprise amember forming a simple spiral passage or an alternative form of passageinclined relative to the conduit axis, can optionally decelerate fluidswithout any inducing any significant turbulent effect.

Optionally, the spiral passage is tightly wound, so that the spiralpassage is longer than the conduit in which it is positioned, andpreferably considerably longer. The angle of the spiral passage in thesetightly wound embodiments can be between 75 degrees and 90 degrees tothe conduit axis. Such embodiments can cause fluids to be decelerateddue to forcing the fluids to continually change direction in the (inuse) horizontal plane orthogonal to the axis. As the fluids travel inthe circular plane, they will typically collide with the outer wall ofthe conduit, or any sleeve or shroud surrounding the passage, and theywill be decelerated by friction between the fluids and that interface.This can be in addition, or instead of, any turbulent effect.

As explained above, embodiments including a spiral passage have theadvantage that gravity assists the flow of fluids along the passage andthat any suspension in the fluids is prevented from settling out, due tothe continuing slope of the passage.

Optionally, the body members connect by interlocking means, which mayinclude tongues and grooves.

Optionally, the at least one body member is cemented or otherwise fittedinside the casing or liner string.

Typically, the apparatus is used in conjunction with equipment, such asa shoe and/or a float collar, at least one of which is provided with avalve (typically a one-way valve). Preferably, the cross-sectional areaof the flow path through the passage is greater than the cross-sectionalarea of the flow path through the valve.

If the valve is provided in the float collar, and in use, the floatcollar is located above the apparatus, then this prevents the apparatusfrom having a choke effect on any fluids passing through it. As the areaof the passage is greater than that of the valve, the passage does notcreate a bigger restriction to the flow of fluid than has already beencreated by the valve and the fluid is not “choked” by the passage.

Thus, in such embodiments, the rate of fluid leaving the shoe and thedeceleration of the fluid is not limited by the cross-section of thepassage, only by the amount of turbulence or other decelerating effectcreated by the apparatus.

Optionally, the apparatus includes at least one collar attached to anend (preferably the lower end) of the casing or liner string, the collarhaving screw threads for attachment to further sections of casing orliner.

The collar can replace the shoe at the (in use) lower end of theapparatus. The collar may couple the casing or liner tubular withinwhich the apparatus is inserted to further casing or other equipment, inthe case that another piece of equipment is required directly above theshoe.

A conventional coupling is typically used to attach the (in use) upperend of the casing or liner tubular within which the apparatus is locatedto the rest of the casing or liner string.

Preferably, the apparatus comprises an anti-rotation means to preventrelative rotation of the body members and thus the passage and the shoe.Typically, the anti-rotation means includes a device, which may be asub, shaped to engage a bore provided in the shoe. Preferably, an axiallocking means is provided to prevent axial separation of the device andthe shoe. Preferably, the axial locking means comprises a latch providedon one of the device and the shoe, and a groove (to engage the latch)provided on the other of the device and the shoe. Most preferably, thelocking means comprises a circlip provided on the device which isadapted to engage a groove in the shoe to prevent axial separation ofthe device and the shoe. Preferably, the anti-rotation means comprises atapered edge provided on one of the device and the shoe and acorrespondingly shaped groove provided on the other of the device andthe shoe. Typically, the tapered edge is provided on the device and thegroove is provided in the shoe. Typically, the anti-rotation meansprevents relative rotation of the at least one body member and the shoeonce the axial locking means has engaged.

The anti-rotation means is useful to help prevent or restrict therotation of the at least one body member and thus the passage when theat least one body member is drilled through. Rotation of the passagewould be disadvantageous as rotation of the drill bit could rotate thepassage, if it is not firmly cemented to the casing, instead of drillingthrough the passage.

Optionally, the apparatus further comprises an outer protection means,which may be a shroud. Typically, the outer protection means is providedwith apertures in the side wall thereof.

According to a second aspect of the present invention there is provideda control assembly, including:

-   -   control apparatus for controlling the flow of fluid into a        borehole through a conduit, the apparatus comprising a        decelerating means adapted to be positioned within the conduit        for slowing down the flow of fluid through the conduit, the        decelerating means comprising a passage in the apparatus;    -   a conduit in which the control apparatus is located; and    -   a valve located in the conduit above the apparatus;    -   wherein the cross-sectional area of the passage in the apparatus        is greater than the cross-sectional area of the valve.

Preferably, the valve is located in a float collar.

According to a third aspect of the present invention there is provided amethod of controlling the passage of fluid through a conduit located ina borehole, including the step of decelerating the fluid.

Optionally, the fluid is decelerated by being passed through adecelerating means located inside the conduit, the decelerating meansbeing adapted to decelerate the fluid passing through the conduit.

Preferably, the decelerating means is inserted into the conduit prior torunning in the conduit into the borehole.

Optionally, the deceleration is caused by the fluid being forced tochange direction. Optionally, the method includes the step of causingthe fluid to deviate from the conduit into a passage which is inclinedrelative to the conduit axis. Some, or all, of the decelerating effectcould be caused by friction as fluid travels along a passage in theapparatus.

Optionally, the fluid travels in a direction having a circularcomponent, which is typically in the (in use) horizontal planeorthogonal to the axial direction.

Typically, the fluid is decelerated by causing it to travel through apassage, which may be a spiral passage, defined by the deceleratingmeans. In use, the inclination of the spiral passage relative to thevertical enables gravity to aid the motion of the fluid through thepassage, and means that any particles suspended in the fluids areunlikely to settle out in the passage to block the passage. The spiralmay be tight, so that fluid will travel through a large distance in asmall axial space.

Optionally, the fluid is decelerated by induction of turbulence into thefluid. This may be achieved by passing the fluid through a spiralpassage including portions spiralling in opposite directions. In suchembodiments, the turbulence may be induced in a connection regionbetween the portions where fluid spiralling in one direction has tochange direction and spiral in the opposite direction.

Typically, the spiral passage includes a plurality of oppositelydirected spiralling portions positioned in series and the fluid passesthrough a plurality of connection regions as it flows through theconduit.

Optionally, the amount of turbulence induced is dependent on the speedof the fluid flow, and the turbulence induced for slowly flowing fluidsmay be zero or negligible.

Typically, a float collar having a valve is provided in the conduitabove the inclined passage, the passage having a greater cross-sectionalarea than the cross-sectional area of the valve so that the fluid flowswithout restriction into the passage.

Typically, a shoe is attached to one end of the conduit, the shoe havinga fluid outlet, and fluid is pumped or passed through the conduit andenters the borehole by the fluid outlet.

Optionally, the inclined passage is defined by at least one body memberhaving formations thereon and a shroud having apertures in its surfaceis provided around the body member, and the method includes the step ofpassing cement through the passage, some of which exits the passage viathe apertures to cement the body member to the conduit.

An embodiment of the invention will now be described by way of exampleonly and with reference to the following drawings, in which:—

FIG. 1 shows a side view with interior detail of two cement toolsstacked on top of each other and inserted in a downhole assembly betweena shoe and a casing string;

FIG. 2 shows a side view with interior detail of the shoe of FIG. 1;

FIG. 3 shows a perspective view of a connector sub of FIG. 1;

FIG. 4 shows a side view with interior detail of a collar which can beused with the tool of FIG. 1;

FIG. 5 shows a side view of a first tool portion;

FIG. 6 shows a side view of a second tool portion;

FIG. 7 shows a plan view of the rear (right hand) end of the second toolportion of FIG. 6, rotated through 180°;

FIG. 8 shows a plan view of the front (left hand) end of the first toolportion of FIG. 5;

FIG. 9 shows a side view with some interior detail exposed of one of thecement tools of FIG. 1;

FIG. 10 shows a schematic diagram of the apparatus assembled in aborehole, with cement forcing the drilling mud through the apparatus;

FIG. 11 shows a schematic diagram of the apparatus with displacementfluid forcing the cement through the apparatus;

FIG. 12 shows a side view with interior detail of an alternativeembodiment of the invention, including a tightly-wound spiral passage;

FIG. 13 shows a schematic diagram of the FIG. 12 embodiment of theinvention located in a casing string between a float collar and a floatshoe; and

FIG. 14 shows a schematic diagram of an alternative arrangement to FIG.13, having a spiral passage spiralling in one direction only.

FIG. 1 shows apparatus in accordance with the present inventioncomprising a first cement tool 10 and a second cement tool 20 coupledtogether. Each tool 10, 20 is made up of a first body member 30 having aleft hand spiral portion and a second body member 40 having a right handspiral portion, shown in FIGS. 5, 6, 7 and 8. It will, however, beappreciated that the left and right hand spiral portions may be swappedwith one another.

The cement tools 10, 20 are located inside a length of casing 60, whichhas standard screw thread connections on each end. The upper end ofcasing 60 is connected to a casing coupling 12 which is attached to therest of the casing string (not shown). It is not necessary for the tools10, 20 to be located inside casing 60; the tools 10, 20 may be locatedinside any conduit which is inserted into the borehole, such asdrillpipe, tubing, coil tubing or liner. The cement tools 10, 20, do notnecessarily extend all the way up the length of casing 60 as shown inFIG. 1; the cement tools 10, 20 typically only extend approximatelyhalfway up the length of casing 60.

Each body member 30, 40 has a central column 31, 41 with a spiralprotrusion 34, 44 extending therefrom. The radially outer edge of thespiral protrusions 34, 44 extends substantially to the inner wall of thecasing 60. Thus, a spiral passage 36, 46 is formed between the surfacesof the spiral protrusion 34, 44, the central column 31, 41 and the innersurface of the casing 60.

The body members 30, 40 are connected together by inter-engaging tonguesand grooves. Each body member 30, 40 has a dove tail or tongue 32 at oneend (here, the upper end with respect to the borehole) and a groove 42in the opposite end. However, in some embodiments, the positions of thetongues 32 and the grooves 42 are reversed. Each tongue 32 isdimensioned so that it is a tolerance fit with its respective groove 42so that the portions 30, 40, will not become accidentally disconnectedin the borehole.

The cement tools 10, 20 are connected together in the same way as thebody members 30, 40; i.e. by connecting the groove 42 of the second bodymember 40 of the first tool 10 with the tongue 32 of the first bodymember 30 of the second tool 20. A connecting passage 86 joins thespiral passages 36, 46 of the body members 30, 40 together, as bestshown in FIG. 9. The connecting passage 86 is preferably cylindrical,having a first axial portion 88 which extends from the (in use lower)end of spiral passage 46, a second axial portion 89 which extends fromthe (in use upper) end of the spiral passage 36 and a third transaxialportion 86A, 86B being a passage travelling through, and across the axisof, the cement tool 10, 20, connecting the first and second axialportions together. The first 88 and second 89 axial passage portions areformed from a pair of off-centre axially arranged cylindrical boresformed respectively through the members 40, 30 and the third transaxialpassage portion 86 is formed from a transaxially arranged cylindricalbore 86 formed through the body members 30, 40 when joined together, sothat the transaxial bore 86 spans the join between the body members 30,40.

In some embodiments, transaxial passage 86 may be inclined relative tothe (in use) horizontal plane, so as to continue the inclined path ofspiral passages 36, 46.

Fluid flowing through the cement tools 10, 20 will be decelerated bybeing forced to change from axial to spiral flow.

The lower end of casing 60 is connected to a shoe 14 by means ofstandard screw threads. The cement tool 10 is connected inside the shoe14 by an anti-rotation connector sub 16 (shown in FIG. 3). The connectorsub 16 has a groove 42 which engages the tongue 32 of the lower end ofthe first cement tool 10. The connector sub 16 has a front portion 54and a rear portion 56. Both portions 54, 56 are cylindrical but portion56 has a larger diameter. The lower end of portion 56 tapers to a pointto provide a tapered end 58. A circlip 62 is disposed in a groove in thefront portion 54.

The shoe 14 has an inner bore shaped to co-operate with the outsidesurface of the connector sub 16. The inner bore has a narrow portion 68with a groove 64 for engagement of the circlip 62. The inner bore of theshoe 14 also has a wider portion 69 having a V-shaped receiving surface70 corresponding to the tapered end 58 to receive the tapered end 58.

The connector sub 16 is inserted into the shoe 14 and, once the circlip62 is aligned with the groove 64 in the inner bore of the shoe 14, thecirclip 62 expands into the groove 64. This prevents further axialmovement between the shoe 14 and the connector 16 (and hence the tools10, 20 and the rest of the apparatus).

The connector sub 16 can be inserted at any angle, as it will alignitself due to the tapered end 58 mating with the V-shaped receivingsurface 70. Once the circlip 62 is engaged, the tapered end 58 cannotescape from the V-shaped receiving surface 70 as the axial movementneeded to do this is prevented by the engaged circlip 62. Furthermore,the connector sub cannot rotate relative to the shoe 14 due to themating of the tapered end 58 and the V-shaped receiving surface 70.Therefore, the shoe 14 is fixed relative to the cement tools 10, 20,both rotationally and axially.

The shoe 14 has a nose 50 having outlet ports 52 to allow fluids to passthrough the shoe 14 into the annulus between the casing and the borehole(not shown). The shoe 14 also typically has a one-way valve 55, toprevent fluids from flowing back into the casing string.

The apparatus is typically used in conjunction with a float collar, asshown in FIGS. 10 and 11. In these figures, casing 60 (in which cementtools 10, 20 are located) is shown coupled beneath a float collar 96.Float collar 96 can be a standard float collar which is commerciallyavailable; such float collars usually include a valve 105, which istypically a one-way valve. For safe operation of the equipment, a valvemust be provided in at least one of the float collar 96 and the shoe 14.

The cross-sectional areas of the respective passages 36, 46 inside thetools 10, 20 are preferably greater than the cross-sectional area of thevalve 105. This means that the fluid flow rate is not limited by thecross-sectional area of the passages 36, 46. The fluid flow rate is onlylimited by the amount of turbulence created inside the tools 10, 20.Therefore the cement tools 10, 20 do not “choke” the fluid, as they donot restrict the cross-sectional area through which it flows.

FIG. 4 shows a collar 80 which can be attached to the cement tool 10,instead of the shoe 14. The collar 80 is typically used in the caseswhere it is not desired to connect the tools 10, 20 directly to the shoe14, e.g. if another tool is required to be inserted above the shoe 14.However, it will also be appreciated that the cement tools 10, 20 couldbe placed at any suitable position in the conduit by any suitablelocating device such as adhesives etc. or even by providing the outerdiameters of the cement tools 10, 20 as a clearance fit with the innerdiameter of the conduit. Each end of the collar 80 is screw threaded forengagement with casing 60 and for engagement with further casing (notshown). The collar 80 has an inner bore similar to that of the shoe 14for engagement with the connector sub 58. The inner bore has a narrowportion 68 with a groove 64 for engagement of the circlip 62 and a wideportion 69, having a tapered circumference 70 corresponding to thetapered end 58. The collar 80 may be used to position the tools 10, 20above the shoe track 93 (the shoe track is shown in FIGS. 10 and 11).(The shoe track 93 is a common term in the industry to designate thecombination of a shoe, one or two joints of casing and a float collar.)

FIG. 9 shows the tool 10 having a shroud 82 around the exterior, whichcould be formed from an easily drillable material. The shroud 82 hasapertures 84 formed in its side wall. The apertures 84 are typicallydistributed throughout the surface of the shroud 82.

The shoe 14, the tools 10, 20, the connector sub 16, any collar 80 andany plugs used with the apparatus are preferably made from materialswhich can be drilled through, such as a plastic or aluminium. The tools10, 20 and connector sub 16 are preferably made out of a thermoplastic.

In use, the shoe 14, connector sub 16, tools 10, 20, casing 60 andcasing coupling 12 are connected to form the assembly shown in FIG. 1 byengaging screw threads, tongues and grooves as described above. Theassembly is then run into the borehole and drilling mud is pumped downthrough the casing string. When the assembly reaches the required depth,the casing is cemented in place. This is done by pumping cement downthrough the casing string. The cement is pumped on top of the drillingmud already in the casing string, and displaces the drilling mud,accelerating the mud down through the casing string and the tools 10,20.

The cement may be pumped directly on top of the drilling mud, in whichcase it could be advantageous to start with a low density cement slurryand to gradually build up the density. Cement additives (commerciallyavailable) have been developed to control the density of the cementslurry. The density can be lowered by adding an additive which has a lowspecific gravity, or which allows large quantities of water (which islighter weight than cement) to be added to the cement, or a combinationof both. The lead slurry should therefore be the lightest; typicallyaround 10 lb/gallon, followed by an intermediate slurry of around 11.5lb/gallon, and a tail slurry of 15 lb/gallon.

In this way, full density cement is not directly on top of the drillingmud, and this reduces the probability of the cement falling through themud. The decelerating action of the tools 10, 20, which will be detailedsubsequently, also reduces the likelihood that the cement will fallthrough the mud.

Alternatively, as shown in FIG. 10, a plug 90 could be positionedbetween the drilling mud 94 and the cement 92. The plug 90 typically hasa sheer section 91 which breaks on the application of a thresholdpressure. In the case where the tools 10, 20 are located directly on topof the shoe 14, the plug 90 lands on top of the float collar 96. FIG. 11shows the plug 90 landed and sheared by the pressure of the cement 92above it. The float collar 96 typically has an anti-rotation device (notshown), such as saw tooth protrusions, to engage the plug 90 and toprevent rotation of the plug 90 when it is subsequently drilled through.

The FIG. 10 embodiment also shows the casing 60 (which contains thecement tools 10, 20) and a following casing string 61 havingcommercially available centralisers 98 to hold the casing 60 and thecasing string 61 in the centre of the borehole 95.

In the case (not shown) where the tools 10, 20 are located above theshoe track 93 such that the tools 10, 20 would be located in the casingstring 61, a landing device (not shown) is typically provided to landthe plug 90. The landing device would typically have an anti-rotationdevice to prevent rotation of the plug, as explained above.

Before the cement puts pressure on the drilling mud, the drilling mudflows slowly enough through the tools 10, 20 for the flow to be laminar.The flow of the drilling mud is not choked by the apparatus, because thecross-sectional areas of passages 36, 46 are greater than thecross-sectional area of the valve 105 in the float collar 96. Thus, thetools 10, 20 do not restrict the flow of the drilling mud before thecement is introduced into the casing string; the only restriction on theflow of the drilling mud is the size of the valve 105.

However, when the mud is accelerated by the cement, the velocity of themud is increased sufficiently for the drilling mud to become turbulent.As the drilling mud passes from the right-hand spiral portion 40 to theleft-hand spiral portion 30, the drilling mud is forced to spiral in theopposite direction. Anticlockwise spiralling mud meets clockwisespiralling mud in the passage 86 between the portions 30, 40 such thateddy currents build up and the mud in the passage becomes turbulent. Theturbulence restricts the flow of the mud through the tools 10, 20. Thus,the velocity of the mud which leaves the shoe and flows up the annulusbetween the casing and the formation is reduced, thereby exerting areduced pressure on the formation and reducing the probability of theformation breaking down.

When the cement reaches the tools 10, 20, some of the cement flowsthrough the apertures 84, which serves to cement the tools 10, 20 to thecasing 60.

Cement is continued to be pumped through the casing string until all thedrilling mud 94 has been expelled from the shoe 14 and the cement 92 nowfills the annulus between the casing string 61 and the borehole 95. Aplug 102 (see FIG. 11) is typically used to act as a separator betweenthe cement 92 and a displacement fluid 100 (e.g. more drilling mud) usedto propel the cement 92 downwards. Typically, this plug 102 lands on thefloat collar 96 (or the landing device, if the tools 10, 20 are locatedabove the float collar 96), on top of any previous plug 90. Thus, whenthe cement 92 sets, in addition to filling the annulus, it will alsofill all of the apparatus below the plug, including the tools 10, 20. Ifdeeper drilling is required, any plugs, the tools 10, 20, any collar 80and the shoe 14 are drilled through.

Modifications and improvements can be made without departing from thescope of the invention. For example, more or fewer tools 10, 20 may beused in combination. The plastic or aluminium shroud 82 and theanti-rotation connector sub 16 are not essential elements of theinvention. For instance, the tools 10, 20 could be cemented into thecasing 60, or otherwise fixed to the casing 60 or the casing coupling12; thus obviating the need for the anti-rotation connector sub 16.

Also, left-hand and right-hand spiral portions 30, 40 need not bepositioned alternately; two portions 30 could be followed by twoportions 40. The tool could optionally comprise only one spiral portion,or a combination of uni-directional spiral portions. In furtheralternative embodiments, the spiral portions 30, 40 could be replaced bya combination of straight axially arranged portions (not shown) andcircumferentially arranged portions (not shown) such that the fluidwould flow around a circumferential portion at one height and then flowsdown the straight axially arranged portion to the next lowercircumferential portion and so on.

Furthermore the spiral portions 30, 40 need not be attached by tonguesand grooves; other attachment means such as screw threads could beprovided. The shoe 14 could be any type of shoe such as a reamer shoe, aguide shoe or a float shoe.

The anti-rotation sub 16 is not an essential feature of the invention.In some embodiments, it is not necessary, e.g. the cement tools 10, 20can be cemented, jammed or secured in any other way to the inside of thecasing or other conduit so as to prevent rotation.

In the case where the cement tools 10, 20 are located inside drillpipe,neither the shoe 14 nor the collar 80 would be necessary. The drillpipecould be hung off (i.e. from a casing string) in such a way as toprevent rotation of the drillpipe. The cement tools 10, 20 could bedimensioned to be a clearance fit inside the drillpipe, to jam the tools10, 20 inside the drillpipe to prevent relative rotation therebetween.

The passage 86 between spiral portions 30 and 40 could include a chamberwider than the rest of the passage in which the streams of oppositelyflowing fluid could meet and interact.

A further modification is shown in FIG. 12, which shows an modifiedcement tool 110 inserted inside a casing length 122. Casing coupling 12is also shown; casing coupling 12 is the same as that shown in FIG. 1,and therefore the same reference number has been used.

Like the cement tools 10, 20 of the FIG. 1 embodiment, cement tool 110has a central column 112 with a spiral protrusion 114 extendingtherefrom.

Spiral protrusions 114 extend substantially to the inner wall of thecasing 122 and define a spiral passage 116 between the surfaces of thespiral protrusion 114, the central column 112 and the inner surface ofcasing 122. The spiral is typically tightly wound, so that spiralpassage 116 is longer than the axial length of cement tool 110. Spiralpassage 116 spirals clockwise when viewed from the (in use) upper end ofcement tool 110.

As in the FIG. 1 embodiment, the spiral passage 116 permits gravity toaid the flow of fluids along the passage, and reduces the chance of anysuspended particles carried by the fluid settling out and blocking thepassage.

It can be beneficial if the cross-sectional area of spiral passage 116is greater than the cross-sectional area of a typical float collarvalve. In such embodiments, the passage 116 does not limit or choke theflow of fluids when used in combination with a float collar having avalve. However, alternative embodiments of the invention can have apassage with a smaller cross-sectional area than that of a float collarvalve.

Although only one cement tool 110 is shown in FIG. 12, it will beappreciated that this could be attached to one or more further cementtools, e.g. by interlocking tongues and grooves, as shown in the FIG. 1embodiment. The further cement tool may have a passage which spirals ina clockwise or anticlockwise direction.

FIG. 13 shows a schematic diagram of an assembly including two types ofcement tool 110, 140. In this embodiment, two lengths of casing 122, 120are connected together between float collar 96 and shoe 14. However, theinvention is not limited to use in conjunction with a either a floatcollar or shoe.

Cement tool 110 is the one shown in detail in FIG. 12. Cement tool 140is similar to cement tool 110, also having spiral protrusions 114 whichdefine a spiral passage 116. However, the direction of spiral passage incement tool 140 is reversed; this passage is spiralling anticlockwisewhen viewed from the (in use) upper end of the cement tool.

A first pair of cement tools 110, 140 are connected together; these arealso connected to a second pair of cement tools 110, 140. In thisembodiment, each cement tool 110, 140 is half as long as a length ofcasing, so that the two pairs of cement tools 110, 140 fill both casinglengths 120, 122. In this schematic diagram, diagonal lines indicate thespiral protrusions 114 and the direction of spiral, but the full detailsof the cement tools 110, 140 are not shown.

However, it will be appreciated that the length of each cement tool 110is not important, and a greater number of shorter cement tools, or asmaller number of longer cement tools could equally be used. A yetalternative arrangement is shown in schematic form in FIG. 14, wherein asingle, longer cement tool 150 is located inside casing length 120.Cement tool 150 is of the same form as cement tool 110 shown in detailin FIG. 12, only longer. Thus, this embodiment causes fluid to spiral inone direction only. In this embodiment, no cement tool is located insidecasing 122, which is empty.

As with the FIG. 1 embodiment, a shroud (see FIG. 9) can optionally beprovided around cement tool 110, although this detail is not shown inFIGS. 12 to 14.

In the embodiments of FIGS. 12 to 14, spiral passage 116 between spiralprotrusions 114 is long and tightly wound. Therefore, the total lengthof spiral passage (i.e. made up of the combined lengths of the passages116 of all of the cement tools 110, 140 used) is considerably longerthan (and may be many times as long as) the length of casing in whichthe cement tools 110, 140 are located.

In use, cement tools 110, 140 are fitted together and assembled insidethe casing lengths 122, 120 as required between float shoe 14 and floatcollar 96. Cement is then pumped down the inside of the casing. Thedetails of this are the same as described above with reference to theprevious embodiment, e.g. the first portion of cement is typically lowdensity cement slurry, and the density is then gradually built up tofull density to reduce the likelihood of the cement “falling through”the drilling mud. Alternatively or additionally, a plug with a sheersection (such as plug 90 in FIG. 10) can be used to keep the cement andthe drilling mud separate until plug 90 lands on float collar 96.

The cement pushes the drilling mud through the cement tools 110, 140.The drilling mud is forced to continually change direction to follow thespiral passage 116. The tighter the spiral, the greater the deceleratingeffect. Friction with the inside of the casing (or optional protectiveshroud) and spiral protrusions 114 decelerates the drilling mud. Thus,the embodiments shown in FIGS. 12 to 14 can decelerate a fluid with orwithout any additional deceleration caused by turbulence.

The drilling mud is propelled out of shoe 14 and up the annulus betweenthe outside of casing lengths 122, 120 and the borehole. However, as itsspeed has been reduced by cement tools 110, 140, the pressure on theformation is eased, rendering the formation less likely to collapse.

1. Apparatus for controlling the flow of fluid into a borehole through aconduit, the apparatus comprising a decelerating means adapted to bepositioned within the conduit for slowing down the flow of fluid throughthe conduit.
 2. Apparatus as claimed in claim 1, wherein thedecelerating means comprises a passage in the apparatus.
 3. Apparatus asclaimed in claim 2, wherein the passage is defined by at least one bodymember having formations thereon.
 4. Apparatus as claimed in claim 3,including a shoe adapted for engagement with the at least one bodymember.
 5. Apparatus as claimed in claim 4, including an anti-rotationmeans to prevent relative rotation of the at least one body member andthe shoe.
 6. Apparatus as claimed in claim 5, wherein the anti-rotationmeans includes a device shaped to engage a bore provided in the shoe. 7.Apparatus as claimed in claim 5, wherein the anti-rotation meanscomprises a tapered edge provided on one of the device and the shoe anda correspondingly shaped groove provided on the other of the device andthe shoe.
 8. Apparatus as claimed in claim 4, including an axial lockingmeans to prevent axial separation of the device and the shoe. 9.Apparatus as claimed in claim 8, wherein the axial locking meanscomprises a latch provided on one of the device and the shoe, and agroove provided on the other of the device and the shoe.
 10. Apparatusas claimed in claim 8 when dependent on claim 5, wherein theanti-rotation means prevents relative rotation of the at least one bodymember and the shoe once the axial locking means has engaged. 11.Apparatus as claimed in claim 3, wherein the apparatus includes a shroudwhich is disposed around the at least one body member.
 12. Apparatus asclaimed in claim 11, wherein the shroud is provided with apertures inthe side wall thereof.
 13. Apparatus as claimed in claim 2, for use inconjunction with equipment having at least one valve, wherein thecross-sectional area of the passage is greater than the cross-sectionalarea of the at least one valve.
 14. Apparatus as claimed in claim 2,wherein the passage has constant dimensions.
 15. Apparatus as claimed inclaim 2, wherein the boundaries of the passage are smooth and free ofobstructions.
 16. Apparatus as claimed in claim 2, wherein the passageis inclined relative to the axis of the conduit and wherein decelerationof the fluid is caused by friction between the fluid and the inclinedpassage.
 17. Apparatus as claimed in claim 2, wherein the passage isinclined relative to a plane perpendicular to the axis of the conduit.18. Apparatus as claimed in claim 16, wherein the inclination of thepassage is continual throughout the length of the passage.
 19. Apparatusas claimed in claim 2, wherein the passage is unidirectional in theaxial direction.
 20. Apparatus as claimed in claim 2, wherein thepassage includes at least one spiral portion.
 21. Apparatus as claimedin claim 20, wherein the angle of the spiral portion of the passage ismore than 60 degrees relative to the axis of the conduit.
 22. Apparatusas claimed in claim 20, wherein the angle of the spiral portion of thepassage is between 70 degrees and 80 degrees relative to the axis of theconduit.
 23. Apparatus as claimed in claim 2, wherein the passageincludes at least one portion which spirals in a first spiral directionand at least one further portion which spirals in a second oppositespiral direction.
 24. Apparatus as claimed in claim 23, wherein a cavityis provided between the at least two oppositely directed spiral passageportions.
 25. Apparatus as claimed in claim 1, wherein the deceleratingmeans is adapted to induce turbulence into the fluid.
 26. Apparatus asclaimed in claim 25, wherein the turbulence is at least partiallyinduced by a direction altering means which causes a change in the flowdirection.
 27. Apparatus as claimed in claim 25 when dependent on claim23, wherein the turbulence is induced in the cavity between the at leasttwo oppositely-directed spiral passage portions.
 28. Apparatus asclaimed in claim 1, wherein the conduit is selected from the groupconsisting of drillpipe, tubing, coiled tubing, filtration screen,casing and liner string.
 29. A control assembly, including: controlapparatus for controlling the flow of fluid into a borehole through aconduit, the apparatus comprising a decelerating means adapted to bepositioned within the conduit for slowing down the flow of fluid throughthe conduit, the decelerating means comprising a passage in theapparatus; a conduit in which the control apparatus is located; and avalve located in the conduit above the apparatus; wherein thecross-sectional area of the passage in the apparatus is greater than thecross-sectional area of the valve.
 30. An assembly as claimed in claim29, wherein the valve is located in a float collar.
 31. A method ofcontrolling the passage of fluid through a conduit located in aborehole, including the step of decelerating the fluid.
 32. A method asclaimed in claim 31, including the step of causing the fluid to deviatefrom the conduit into a passage which is inclined relative to theconduit axis.
 33. A method as claimed in claim 32, wherein the fluid isdecelerated by friction between the fluid and the boundaries of theinclined passage.
 34. A method as claimed in claim 32, wherein theinclined passage has constant dimensions and the boundaries of thepassage are free of obstructions so that the fluid moves along thepassage without hindrance.
 35. A method as claimed in claim 31,including the step of causing the fluid to travel in a spiral direction.36. A method as claimed in claim 35, wherein the fluid is caused totravel in a tight spiral so that it travels through a large distance ina small axial space.
 37. A method as claimed in claim 35, wherein thefluid is caused to travel in a first spiral direction and subsequentlyin a second opposite spiral direction.
 38. A method as claimed in claim32, wherein a float collar having a valve is provided in the conduitabove the inclined passage, and wherein the passage has a greatercross-sectional area than the cross-sectional area of the valve so thatthe fluid flows without restriction into the passage.
 39. A method asclaimed in claim 31, including the step of inducing turbulence into thefluid.
 40. A method as claimed in claim 37, wherein turbulence isinduced by causing the fluid to change direction from the first spiraldirection to the second spiral direction.
 41. A method as claimed inclaim 32, wherein the inclined passage is defined by at least one bodymember having formations thereon and wherein a shroud having aperturesin its surface is provided around the body member, the method includingthe step of passing cement through the passage, so that some of thecement exits the passage via the apertures to cement the body member tothe conduit.